Guys, I think this fellow is talking about the Female threaded bolt hole, which is located between the 3rd and 4th exhaust ports on the Drivers side. Is that not the steering box I see in the back ground.
Not sure if the Bird exhaust casting is used on other cars, (to lazy to search the Blue book) but it would be for bolting up a brace for possible A/C brackets, or inserting a lifting eye/guide fixture for assembly. There is a threaded hole also in the passenger exhaust casting, in the same front to back dimension.
No need to fill it or cover it, it is a blind hole.
There is no reference in the Osborne Engine Equipment Assembly Manual, of these threaded holes or brackets mounted.

Here's a pic of the a/c brace on my car. Never looked at the passenger side but Alan is right there is a hole there also so must have been a dual purpose hole. I'm not sure I would trust them with the weight of the engine and trans though.

David, you would be amazed at the tensil strength that a tight 3/8 bolt has under tension. You would be able to virtually pick up the whole car by it.
As I mentioned these holes were for fabrication and bracketry and were located on both sides. I never tried to pick up an engine by these; to see if their location were a centroid point of balance. Interesting concept though.